1. Field of the Invention
The present invention relates to a production method for oxygen, in which high-pressure oxygen gas is produced by compressing and heating liquid oxygen which is obtained by cryogenic distillation, etc.
2. Description of the Related Art
In a typical production method for high-pressure oxygen, low-pressure oxygen is first obtained, and is then compressed using an oxygen compressor.
With this method, however, there is a safety hazard in that reactivity between the oxygen and the material of the compressor will be high since the temperature of the oxygen is increased by the heat from the compression. In addition, maintenance costs, as well as cost for equipment, are high.
On the other hand, to avoid this, another method is also known in which liquid oxygen obtained by an air separation unit is compressed, and is then heated by a heat exchanger.
Conventionally, in this method, the liquid oxygen is compressed by a pump and is then evaporated by exchanging heat with hot stream, for example, compressed raw air, in a brazed aluminum plate-fin heat exchanger. This method will be referred to as a conventional compression method in the following descriptions.
The brazed aluminum plate-fin heat exchanger provides excellent heat conductivity and may be used for multiple fluids. In addition, the equipment is compact relative to the heating area thereof and can be provided at low cost. Accordingly, the brazed aluminum plate-fin heat exchanger is a key piece of hardware in the conventional compression method.
The brazed aluminum plate-fin heat exchanger, however, is not sufficiently resistant to cyclic stress because of its brazed construction. From the viewpoint of protecting the brazed aluminum plate-fin heat exchanger, it is necessary to reduce the amount of stress which occurs therein. Thus, the brazed aluminum plate-fin heat exchanger has not been used in process to produce high-pressure oxygen.
Accordingly, when high-pressure oxygen is required, the conventional compression method is used to increase the pressure of the oxygen to 3.5 MPa at most, and further compression is performed by the oxygen compressor.
As a result, the amount of stress that occurs in the heat exchanger is reduced; however, since the oxygen compressor is used, the above-described problems of safety hazards and high cost remain. Accordingly, there has been a demand to solve such problems.
Accordingly, it is an object of the present invention to provide a production method for oxygen, in which the conventional compression method which is advantageous regarding cost is used, and in which thermal stress occurs in the heat exchanger is reduced, so that the pressure of the oxygen may be safely increased to a desired level.
According to a production method for oxygen of the present invention, liquid oxygen is compressed so that the pressure thereof exceeds the critical pressure, and is then drawn into a plate-fin heat exchanger as cold stream. The liquid oxygen is heated in the plate-fin heat exchanger so that the temperature thereof exceeds the critical temperature and is then taken out from the plate-fin heat exchanger.
According to the method, the pressure of the liquid oxygen, which signifies oxygen-rich liquid, is increased to exceed the critical pressure (5.043 MPa). The liquid oxygen is then led into the plate-fin heat exchanger, which may be brazed aluminum plate-fin heat exchanger, in which the temperature thereof is increased to exceed the critical temperature. Thus, the oxygen becomes a supercritical fluid in the heating process, and a phase change in the oxygen does not occur in the heat exchanger.
To describe this more specifically with reference to FIG. 2, when cold stream A, in which the pressure is lower than the critical pressure, is heated, there is a state in which the fluid A evaporates while the temperature thereof does not change much due to the latent heat.
In contrast, when cold stream B, in which the pressure is higher than the critical pressure, is heated, there is no boiling point or the latent heat, so that the fluid B becomes a supercritical fluid. In supercritical fluids, there is no evaporation, so that phase change does not occur. Thus, the temperature of cold stream B smoothly increases along with the amount of the heat with the hot stream.
The temperature profile inside the heat exchanger is determined by the temperature of each fluid. As shown in FIG. 3, when the pressure of the cold stream is lower than the critical pressure, the temperature difference between the cold stream and the hot stream is large. Accordingly, there is a risk that the difference in amounts of heat shrinkage between members of the heat exchanger will cause a great amount of thermal stress so as to damage the heat exchanger.
On the other hand, as shown in FIG. 4, with the fluid in which the pressure is higher than the critical pressure, the temperature difference xcex94t is small, so that the thermal stress is also small. Thus, even a relatively weak heat exchanger may be used.
Accordingly, the conventional compression method which is advantageous regarding cost may be used while the safety of the heat exchanger, for example, a brazed aluminum plate-fin heat exchanger, is ensured, and the desired high-pressure oxygen will still be obtained.
Especially when the pressure of the liquid oxygen is higher than 8.049 MPa, which far exceeds the critical pressure, stable operation is realized since the operating pressure is higher than the pressure loss in the system. Accordingly, the supercritical fluid is more stable, so that the effect of reducing stress in the heat exchanger is enhanced.
The flow rate of the oxygen in the heat exchanger is preferably not more than 5 m/sec which is the standard flow rate for safety (the lower limit is 0.5 m/sec). Accordingly, the heat exchange of the oxygen is safely performed.
In addition, the temperature difference between hot stream and cold stream in the heat exchanger is preferably not more than 20xc2x0 C. Accordingly, the stress occurs in the heat exchanger is reduced.
As described above, thermal stress is not caused by a phase change in the heat exchanger. Thus, even when a load change occurs due to, for example, differences in oxygen flow rates between day and night, the heat exchanger may be sufficiently resistant against stress occurs therein.
Accordingly, the heat exchanger may be continuously operated safely even under conditions in which a relatively high degree of load variation occurs.
The liquid oxygen which is to undergo the compression and heating process may be obtained by the air separation unit. In such a case, high-pressure oxygen is obtained in one of the processes (a process of increasing internal pressure) performed in the air separation unit, so that no additional equipment is required. Accordingly, the cost of equipment may be reduced, and oxygen may be produced at higher efficiency and at lower cost.
Raw air required as a material in the air separation unit is preferably compressed so that the pressure thereof exceeds the critical pressure. In addition, the balance between the pressure and the flow rate of the raw air is preferably adjusted before it is used. Accordingly, the temperature difference between the raw air and cold stream, in which the pressure is higher than the critical pressure, may be extremely low. Thus, the amount of local stress may be extremely small.